In this article we will discuss about how to measure the consumptive use of water for irrigation.
Methods of Direct Measurement of Consumptive Use:
The following are the five methods which can be used to measure the consumptive use of water:
1. Inflow and outflow studies
2. Tank and lysimeter method
3. Soil moisture studies
4. Field experimental plots
5. Integration method.
Each method has been explained here in brief:
1. Inflow and Outflow Studies:
This method is useful for comparatively larger areas. It gives annual consumptive use.
The value of annual consumptive use may be found using following equation:
All these volumes have to be measured in hectare-metre.
2. Tank and Lysimeter Method:
3 m deep and 10 m square tanks are set flush with the ground level. It is good if tanks of larger area are adopted as they provide better resemblance to root development. In order to obtain satisfactory proper growth of the plant constant moisture conditions are maintained within the tank.
The quantity of water required to maintain constant moisture conditions within the tank is the consumptive use of water. In Lysimeters the bottom of the tank is made pervious. Some of the water applied in the tank is used as consumptive use and the remaining part of water drains through the pervious bottom and gets collected in a pan. The difference between water applied and the water collected in the pan is known as consumptive use of water.
3. Soil-Moisture Studies:
This method is found suitable where soil is fairly uniform and ground water is deep enough that it does not affect the soil moisture within the root zone of the soil. Soil moisture measurements are done before and after each irrigation. The quantity of water extracted per day from the soil is determined for each period. A curve is plotted between rate of use and time. This curve gives seasonal use.
4. Field Experimental Plots:
Irrigation water is applied to the field experimental plot in such a way that there is neither run-off nor deep percolation. Yield obtained from different fields are plotted against the total water used. The curve so obtained forms the basis for determination of the consumptive use it is seen that for every type of crops, the yield increases rapidly with an increase of water used to a certain point, and then starts decreasing with further increase in water. The break point in the curve reflects amount of water which is consumptive use of water. This method is better than tank and Lysimeter method.
5. Integration Method:
In this method the total area is divided into following areas:
1. Areas under irrigation for different crops
2. Areas under natural vegetation
3. Area under water surface
4. Area under bare land.
This method consists summation of the products of the above mentioned different areas with their respective unit consumptive uses. The summation of the results is known as annual consumptive use for the whole area. It is measured in hectare-metre units.
Methods of Determination of Consumptive Use of Water by Use of Equations:
Following three methods are commonly used:
1. Penman-method
2. Blaney-Criddle method
3. Hargreaves class A pan evaporation method.
1. Penman Method:
Consumptive use of water is determined by the following equation –
The above given equations are quite complex.
In order to facilitate calculation work several factors are determined as follows:
RA can be determined from Table 3.1,
σT4a can be read from Table 3.2.
The saturation vapour pressure (ea) can be found from Fig. 3.4 given here. Δ can be read from Fig. 3.5. Tables 3.1 and 3.2 as well as Fig 3.4 and 3.5 were developed by Mr. Criddle. He suggested a Tabular form for the systematic solution of Panman equation.
2. Blaney-Criddle Method:
Blaney-Criddle gave following equation which expresses the consumptive use of water in terms of temperature and day time hours –
3. Hargreaves Class A Pan Evaporation Method:
Consumptive use or Evapo-Transpiration of water is given by following equation –
ɸCu or Et = k Ep
Irrigation Efficiency:
The existing water resources of the country are adequate to irrigate only 50% of the cultivable area. This aspect makes it imperative for Indian engineers to devise ways and means to conserve water and make its judicious and economical use in irrigation.
Studies conducted on canal irrigation projects located in Northern India reveal that 15-20% of the water supplied from diversion head works is lost in mass and branch canals only, 6 to 8% losses occur is major and minor distributions, losses in field channels amount from 20 to 22%, so we see that 41 % to 50 % water supplied from diversion head works is lost only in conveyance before it reaches the fields for irrigation but of remaining about 50%, water supplied to fields, about 22% is further lost due to surface evaporation, deep percolation, and irregular distribution. Thus the effective percentage of water that is utilised by the crops is the form of Evapo-Transpiration or consumptive use, is only 28-29% of the total water released in the main canal from diversion head works.
Due to permeability of soils, Topography and climatic conditions, it is impossible to achieve 100% utilisation of water for the growth of crops. But the above said losses in conveyance and other means suggest ample scope of improvement in the efficiency of irrigation. With correct management and by adopting such measures that would cause least losses, reasonably high efficiency can definitely be achieved.
Efficient use of irrigation water is expected from each user as well as the planners. Even under the best method of irrigation not all the water applied during irrigation is stored in the root zone. The efficiency of irrigation water is the ratio of water output to the water input and is expressed as percentage.
The following are the various types of irrigation efficiencies:
1. Water conveyance efficiency
2. Water application efficiency
3. Water use efficiency
4. Water storage efficiency
5. Water distribution efficiency
6. Consumptive use efficiency and
7. Project efficiency.
1. Water Conveyance Efficiency:
This terms indicates the ratio of irrigation water that is available at the outlets to the fields, to irrigation water supplied from the diversion point into the main canal.
Ƞc = (Wf / Wr)x 100
Where
Ƞc = Water conveyance efficiency
Wf = Water delivered to the farm or plot
Wr = Water diverted from the river or reservoir into the main canal.
2. Water Application Efficiency:
It is the ratio of the quantity of water stored into the root zone of the crops to the quantity of water delivered to the field.
It is expressed as follows –
ƞc =(Ws / Wf)x 100
Where
ƞa = Water application efficiency
Ws = Water stored in the root zone during the irrigation
Wf = Water delivered to the farm or plot.
The loss of irrigation water during water application is by I surface run off Rf from the farm and II deep percolation Df below the farm root-zone soil.
Following are the factors which responsible for low water application efficiency:
i. Irrigation land surfaces
ii. Irrigation streams being either very small or excessively large
iii. Shallow soils underlain by gravels of light permeability
iv. Wrong irrigation methods
v. Field preparation not being proper
vi. Compact impervious soil
vii. Steep slopes of land surfaces
viii. Excessive single application of water
ix. Long irrigation runs
x. Careless attendance during water application to the fields.
In a well-designed surface irrigation system water application efficiency should at least be about 60% and that for sprinkler irrigation system about 75%. This efficiency focuses the attention of the suitability of the method of application of water to the crops.
3. Water Use Efficiency:
Water use efficiency is the ratio of water beneficially used including leaching water, to the quantity of water delivered. It is expressed as follows –
4. Water Storage Efficiency:
ƞs = (Ws / Wƞ) x 100
Where
ƞs = Water storage efficiency
Ws = Water stored in the root zone during irrigation
Wh = Water required in the root zone prior to irrigations
= (Field capacity – Available moisture)
This efficiency gives an insight to how completely the required water has been stored in the root zone during irrigation.
5. Water Distribution Efficiency:
Water distribution efficiency is determined from the following equation
ƞd = 100 [1 – y/d]
Where
ƞd = Water distribution efficiency
y = Average numerical deviation in depth of water stored from average depth stored during irrigation
d = Average depth of water stored during irrigation.
This efficiency evaluates the degree to which water is uniformly distributed throughout the root zone. The more uniformly the water is distributed, the better will be the crop response. This efficiency also provides a measure for comparing various methods or systems of water application.
6. Consumptive Use Efficiency:
It is defined by following ratio –
This efficiency evaluates the loss of water by deep percolation and by excessive surface evaporation after an irrigation.
7. Project Efficiency (ƞp):
ƞp = [Wcu / Wr]x 100
ƞp = Project efficiency
Wcu = Amount of water used in form of consumptive use by the crop
Wr = Amount of water supplied from diversion head works
We see that no efficiency is 100%. This is due to losses of irrigation water by many ways. If somehow these losses are reduced, the project efficiency can be considerably increased. These losses can be minimised by liming of canals and water courses, by designing deep narrow canals, by preparing the fields properly and by adopting so many other measures by which loss of irrigation water is reduced.